An option to consider is a single 18" instead of two 15's. Single sub is cheaper than two and weighs less. Single sub is also better if you factor in time alignment. Drawback would be potential power compression.

SSA Xcon comes to mind, and you can give it 3000 wrms. SQ is relative to your two options and it can dish out some retarded db's. And it sounds like this is what you're looking for.

All of these options are boomy/muddy, so some may not even consider them in SQ category like DLS, Dynaudio, etc - well known for their transparency and lower distortion levels, but lacking in vein shattering and lung collapsing experience your options can give you. As for SQ, all your options are lateral and the difference will be miniscule at best, eventhough Alpine Type X is more on par with the Q, not the R.